1. Field of the Invention
This invention relates to a novel power combiner arrangement for microwave power amplifiers, and more particularly, to a lossless power combiner in conjunction with a plurality of sensing devices, each sensing device disposed at a separate one of the input ports to the combiner, the sensing devices being able to identify and remove failed amplifiers at the input to the combiner and thereby improving the degradation performance of the system.
2. Description of the Prior Art
Solid-state microwave power amplifier units have power capacity too low for the output requirements of radar and communication systems. A number of different techniques for providing high power amplifiers for these systems are known in the prior art which overcome these power limitations imposed by individual solid state amplifier elements. A basic requirement of all such techniques, besides the obvious one of obtaining the combined output power of all the individual amplifiers, is for the total output power to fall gracefully as one or more of the component amplifiers suffer arbitrary modes of failure.
One such arrangement is discussed in the article "An N-Way Hybrid Power Divider" by E. J. Wilkinson in IRE Transactions on Microwave Theory and Techniques, Vol. MTT-8, No. 1, January 1960, at pp. 116-118. In the Wilkinson hybrid divider/combiner arrangement, the input ports are isolated from one another, as are the output ports. Therefore, when one or more amplifiers fail, the input and output impedances of each of the remaining amplifiers are unchanged.
A device capable of minimizing the degradation of the combined output power of microwave amplifiers upon such failure is disclosed in U.S. Pat. No. 3,928,806 issued to J. L. Carter et al on Dec. 23, 1975, which relates to a non-reciprocal power combiner. The combiner comprises N isolator-mismatch devices, each coupled to the output of one of a plurality of N microwave amplifiers, and a circulator coupled to the outputs of all of the mismatch devices. Each mismatch device is a two-port device that transmits power in one direction only, into the circulator. No reflected power is returned through any isolator-mismatch device to the amplifier, instead, any reflected power is absorbed by the mismatch device.
An alternative design is disclosed in U.S. Pat. No. 4,010,426 issued to S. I. Rambo on Mar. 1, 1977, which relates to an RF power amplifier redundant system employing three amplifiers. Under normal operation when all three amplifiers are functioning, the output from a comparator holds a 0 degree, 60 degree two-bit phase shifter in the 60 degree position. When any one of the amplifiers or any pair of amplifiers fail, the comparator output causes the two-bit phase shifter to switch to the 0 degree position. The system holds the power output constant at one-fourth the value of each individual amplifier for single or double failures. This one-fourth level, however, is undesirable in most communications applications.
A combiner capable of maintaining a higher power output level upon component failure is disclosed in U.S. Pat. No. 4,064,464 issued to A. W. Morse on Dec. 20, 1977, which relates to an amplitude stabilized power amplifier. A power splitter divides an input signal among a plurality of amplification channels, each having a power amplifier module. The separately amplified signals of the power channels are then recombined in a power combiner to provide the amplified power output signal. One of the power channels is also provided with a voltage controlled phase shifter whose phase shift is controlled by an operational amplifier which is responsive to a reference voltage and also responsive to the power level of the amplified output signal. The control amplifier controls the phase shift of the voltage controlled phase shifter such that when the power of the power amplifier output signal deviates from a predetermined level due to a failure along one of the amplifier channels, the operational amplifier causes the phase shifter to change the phase of the signal in its amplification channel. By this phase shift, the phase shifter affects the power combining efficiency of the power combiner to maintain the power of the power amplifier output signal at the predetermined level.
It has been suggested by R. L. Ernst in an article entitled "Graceful Degradation Properties of Matched N-Port Power Amplifier Combiners" in 1977 IEEE MTT-S International Microwave Symposium Digest, June, 1977 at pp. 174-177 that overall system performance may be significantly improved by replacing each failed amplifier with a "through" connection having the same phase delay as a working amplifier. The problem with this scheme, however, is in the practical difficulty of replacing a failed amplifier with an equal-phase "through" connection.
The problem remaining in the prior art, then, is to provide a means for improving the degradation performance of a microwave power amplifier that is both easy to implement and provides an output power level sufficient for radar and communication system applications.